Astronomical optics require very high precision mirrors, since the maximum surface defect which can be tolerated in a reflecting optical component for use with visible spectrum light is about 0.135 micrometers.
Such mirrors are conventionally obtained from an optical component which is accurately machined and which is referred to as a "substrate", which optical component is then subjected to a subsequent process including the application of a reflecting coating. Such a reflecting coating is obtained in a manner known per se by depositing aluminum in a vacuum.
Obtaining mirror substrates, and consequently mirrors, with a sufficient degree of accuracy is made all the more difficult in that the mirrors used in astronomical optics are circular mirrors having a diameter of one meter or more. The machining of large diameter mirrors is constrained by phenomena concerned with the mirror substrate bending under its own weight because of its large size.
Another difficulty encountered with large diameter mirrors or mirror substrates stems directly from the weight of such mirrors or substrates in that they require particularly strong mechanical means for manipulating them, and such means are consequently expensive.
Another problem encountered in the manufacture of optically precise mirrors or mirror substrates is that the optical qualities of the mirror can only be tested after manufacture has been completed.
The first large diameter precision mirrors were obtained by casting a disk of glass. In addition to the problems associated with making such a large casting, it should be emphasized that mirrors obtained in this way are very heavy because of the high density of glass, which density lies in the range 2 and 2.5 as a general rule.
Other proposals have since been made to remedy these drawbacks, and in particular to obtain lighter mirrors.
Thus, cellular mirrors have been proposed comprising two plates of metal which are interconnected by an intermediate metal structure which is welded to the respective inside faces of the two metal plates. The outside face of only one of the two metal plates is then treated in order to constitute a mirror, with the other metal plate serving solely as a support member. Mirrors made in this way are non-uniform in structure and have never given good images.
The same principle was then considered in an application where the metal structure was replaced by a glass structure which was glued to the respective inside faces of two plates of glass.
This proposition suffered from numerous problems concerned with the gluing, and failed to provide satisfactory images either.
Another solution which was proposed for manufacturing lightened mirrors for the Mount Palomar telescope consisted in directly casting a suitably shaped structure, however the difficulties encountered in practice were very large.
Finally, it must be emphasized that current metal mirrors cannot be used successfully in astronomical optics. As for cellular mirrors whose structure is machined from a block of glass, the manufacturing costs are very high because of the amount of machining required and the difficulty of the machining that is to be performed.
One of the aims of the present invention is to provide a mirror substrate which is as uniform as possible and which is capable of providing a very high degree of optical accuracy, e.g. suitable for astronomical optics.
Another aim of the invention is to provide such an optically accurate substrate for a mirror which may be very large in size.
Another aim of the invention is to provide such a substrate of sufficient mechanical and thermal uniformity in order to remain within the limits imposed by the maximum size of defect which is tolerable for astronomical reflecting optics, namely 0.135 micrometers.
Another aim of the invention is to provide such a mirror substrate which is particularly light in weight and cheap to manufacture.